Improvements in dewaterability and fuel properties of hydrochars derived from hydrothermal co-carbonization of sewage sludge and organic waste

The hydrothermal co-carbonization of sewage sludge and organic additives, namely 10 and 20% of charcoal, fir, grass, and an undersieved fraction of municipal solid waste, was studied. The benefits of this combined process included the spectacular dewaterability performance of slurry, proved by positive filtration tests and shorter capillary suction times. For instance, a 20% fir addition decreased c.a. 60% of pressure filtration time when compared to the hydrothermal carbonization of sewage sludge. A 10% undersieved fraction of municipal solid waste resulted in 15.72 s of capillary suction time. Moreover, hydrothermal co-carbonization produced effective solid energy sources. The addition of organic origin waste to sewage sludge prior to the process caused higher heating values, carbon and fixed carbon contents of hydrochars (e.g. a 20% charcoal addition generated 21% higher heating value, 30% carbon and 2.8 times higher fixed carbon), which corresponded with easier and more stable combustion processes compared to hydrochar from sewage sludge determined by thermal analysis. Possible exploitation problems during combustion have been assessed by determining the tendency risks of slagging and fouling based on oxides identified in ash by XRF analysis. Furthermore, changes in the structural and morphological properties of hydrochars were identified by SEM and FTIR analyses.